Safety of Expressions It is possible to write tuple calculus expressions that generate infinite relations. For example,{tter}results in an infinite relation if the domain of any attribute of relation r is infinite To guard against the problem,we restrict the set of allowable expressions to safe expressions. An expression {t P(t )in the tuple relational calculus is safe if every component of t appears in one of the relations,tuples,or constants that appear in P NOTE:this is more than just a syntax condition. E.g.f f [A]=5 v true is not safe---it defines an infinite set with attribute values that do not appear in any relation or tuples or constants in P. Database System Concepts-6th Edition 6.19 @Silberschatz,Korth and SudarshanDatabase System Concepts - 6 6.19 ©Silberschatz, Korth and Sudarshan th Edition Safety of Expressions It is possible to write tuple calculus expressions that generate infinite relations. For example, { t |  t  r } results in an infinite relation if the domain of any attribute of relation r is infinite To guard against the problem, we restrict the set of allowable expressions to safe expressions. An expression {t | P (t )} in the tuple relational calculus is safe if every component of t appears in one of the relations, tuples, or constants that appear in P NOTE: this is more than just a syntax condition.  E.g. { t | t [A] = 5  true } is not safe --- it defines an infinite set with attribute values that do not appear in any relation or tuples or constants in P